Force-feedback devices are known, in particular in the field of orthoses or of servo manipulators. In various systems in which the user manipulates a control lever in order to impose movements on an arm or any other controlled device, it is advantageous for the user to feel forces in the lever that are representative of the forces acting on the controlled arm. For this purpose, force-feedback devices are used that are coupled to the control lever and that exert thereon a force that is representative of the force to which the controlled arm is subjected.
In the particular field of servo manipulators controlled by a master arm that is manipulated by the user, it is known to fit the master arm with actuators, each including a motor and gearbox unit that acts on a transmission cable.
The drive system constituted by the unit and the transmission suffers from internal friction that it is difficult to reduce to below 5% of the maximum static force of the actuator. Such friction, due in particular to internal friction in the motor and gearbox unit, can disturb the sensations of the user, since it introduces discontinuities in the movements of the arm.
It is known to implement compensation of the friction that opposes free maneuvering of the arm by controlling the motor and gearbox unit so that it exerts a force that compensates said friction, at least in part, and in particular compensates the friction internal to the unit. Nevertheless, such compensation is generally implemented only once it has been detected that the cable has actually been set into movement and that the movable portions of the unit that are coupled to the cable have been moved, which assumes that the user has already exerted a force that is sufficient to overcome at least the internal friction of the unit.